1. Field of the Invention
The present invention concerns computer-related and/or assisted methods, systems and computer readable mediums for metrology during process control. More specifically, it relates to dynamic adjustment of metrology schemes and sampling during advanced process control methods, for example during control of semiconductor technology manufacture.
2. Related Art
In the wafer fabrication art, measurements are made by metrology tools on wafers as they are being manufactured by processing devices, in order to ensure that the wafers are produced according to a predefined specification. The measurements are made of physical properties such as film thickness and uniformity, dopant concentration, gate length and critical dimension. This is known as the science of “metrology.”
Measurements to be made are typically specified in a “die map”. The die map indicates where the different chips (or die) are located on a wafer (in the typical situation where multiple chips are formed on and eventually cut from a single wafer), as well as significant locations, such as corners, on each die. In order to measure the right hand corner on each die, for example, multiple points are measured on the wafer in accordance with the die map. Ordinarily a die map is a digital representation of coordinate points, or “metrology coordinates,” on the wafer.
The metrology coordinates are usually provided by an engineer, and vary depending on the engineer's preferences. Metrology coordinates are conventionally provided as x, y coordinates.
A “sampling plan,” alternatively referred to as a “metrology plan,” contains metrology coordinates drawn from the die map. The sampling plan denotes a specific plan for taking certain measurements. These measurements may include some or all of the possible points and/or chips in the die map.
A conventional metrology system assigns a sampling plan that predetermines which wafers are to be measured in connection with a processing device, and the measurements which are to be taken of those wafers by the metrology tool. For example, the sampling plan might define that each fifth wafer should be measured at pre-designated locations. These sampling plans are not changed after being initially assigned, and hence the metrology systems are static.
Unfortunately, manufacturing results tend to drift away from the intended target or specification when there is a change in the manufacturing process, such as a change in recipe, preventative maintenance, consumables change, environmental change or a new lot of wafers. Conventional metrology systems tend to miss some wafers which are outside specification limits, since these systems use a virtually consistent measurement scheme, having consistently frequent measurements with consistent spatial resolution, without taking into consideration whether any changes were introduced into the manufacturing process which might change the manufacturing results.
Manufacturing systems do not typically call for a measurement of every wafer, since measuring takes time and increasing the number of measurements results in a decrease of productivity. On the other hand, measuring fewer wafers tends to lead to delayed detection of critical information for process control that may significantly impact wafer yield. While conventional sampling systems will sample wafers during and/or after production, these systems do not adjust the initially assigned sampling plan for the wafers during production.
Thus, there remains a need for dynamic metrology to improve the quality of products. For semiconductor wafers, there remains a need to better check whether each specification is met under production conditions. There also remains a need to respond to a change in parameters which may cause a variance from intended target results, such as recipe parameters, and to adjust the frequency and/or spatial resolution of measurements. Unfortunately, taking measurements takes time, and most processing devices are faster than the measurements that need to be taken by metrology tools in order to characterize the wafers using a metrology. Thus, there remains a need for a method, system and medium to react to changes potentially affecting the system results, and to appropriately adjust, increase, or decrease the measurements accordingly.